I believe the weather rss feed only accepts two query parameters to change the output - location and temperature units (as listed here: http://developer.yahoo.com/weather/#params). There is an option in the returned data that displays the language that the forecast is in but I don't see a method for manipulating the output with a language type.
Jonathan LeBlanc Senior Software Engineer Yahoo! Developer Network
A lot of the translation utilities might not give you the most accurate of translations but they'll do a fair job. One of the better language translation APIs I've seen around is the Google's AJAX language API: http://code.google.com/apis/ajaxlanguage/documentation/. I haven't used it myself but it looks fairly straightforward.